Try to flush DOMStorage too when ending the browsing session.

content/public/browser/storage_partition.h

Index: content/public/browser/storage_partition.h
diff --git a/content/public/browser/storage_partition.h b/content/public/browser/storage_partition.h
index 4d4021d8a625eaf1d88654487faacb0d379c4a47..89dcbd5013d25957a39585ba02ee69edde413be5 100644
--- a/content/public/browser/storage_partition.h
+++ b/content/public/browser/storage_partition.h
@@ -8,6 +8,7 @@
 #include <stdint.h>
 
 #include <string>
+#include <vector>
 
 #include "base/callback_forward.h"
 #include "base/files/file_path.h"
@@ -18,6 +19,7 @@
 class GURL;
 
 namespace base {
+class SequencedTaskRunner;
 class Time;
 }
 
@@ -171,10 +173,11 @@ class CONTENT_EXPORT StoragePartition {
       const base::Callback<bool(const GURL&)>& url_matcher,
       const base::Closure& callback) = 0;
 
-  // Write any unwritten data to disk.
+  // Write any unwritten data to disk, returns a collection of task runners
+  // that have been tasked with writing data.
   // Note: this method does not sync the data - it only ensures that any
   // unwritten data has been written out to the filesystem.
-  virtual void Flush() = 0;
+  virtual std::vector<scoped_refptr<base::SequencedTaskRunner>> Flush() = 0;
 
   // Clear the bluetooth allowed devices map. For test use only.
   virtual void ClearBluetoothAllowedDevicesMapForTesting() = 0;